The Ultimate Question of Analog Circuit Design

The 1978 comedy science-fiction work Hitchhiker’s Guide to the Galaxy describes a computer that was specially constructed to determine the “Answer to the Ultimate Question of Life, The Universe, and Everything.” After 7.5 million years of processing (remember, this was written in 1978 when computers were pretty slow), the computer determined that the answer was "42." However, the computer was unable to explain that response to its creators and would need to build a new computer (called "Earth") to compute the explanation.

At my previous employer, we had a designated New Product Coordinator. I won’t embarrass her by using her real name, but it’s the same as a popular flower. I’ll call her “Violet.” Her job was to keep track of where new-product lots were in the manufacturing line, and keep them moving so we’d stay on schedule. She had to work with all aspects of manufacturing to make that happen, including splitting lots for process corners, holding lots at poly or metal for possible changes, etc. She also had to pester the engineers to finish their simulations, then characterizations, etc., in preparation for our weekly new-product reviews.

Management relied on Violet to push the development process hard, to speed it up. She kept the schedules, had all the commitment dates, and had the full backing of all the managers when she asked why someone missed a deadline.

Many an engineer, especially among the younger ones, would get his first chip back from assembly, run to his bench, put a chip in a test fixture, and see if it worked: a quick look for instability, turn up the clock or input signal frequency to see where the chip stopped working, load the output past the spec, then declare success to the rest of the lab -- usually to thunderous applause and high-fives all around. Then a quick write-up to distribute at the weekly review meeting.

Violet did not have an engineering degree and was not a design engineer, but she quickly learned the seven-word Ultimate Question of Analog Circuit Design that every one of our design engineers would learn to dread hearing at a review meeting:

“Did you look at it over temperature?”

Either on the bench with assembled parts or in simulation, the question needed to be asked and Violet knew just when to ask it. “Uhhh… no. But this is a commercial temp device, and I’ve used all these subcircuits and this process lots of times before. Nothing can go wrong” was not an acceptable answer, although some engineers tried it. Few made the same mistake twice.

Violet is retired now. But perhaps someone reading this will get the benefit her genius by asking himself The Ultimate Question before the next tapeout.

Victoria was right on her stress upon the temperature. Even though the process is trimmed out, if the device remains ON say even for few hours then the bias would change and can cause short term failures.

HTOL (High Temperature Operating Life, performed to determine the reliability of devices under operation at high temperature conditions over an extended period of time) is one test that is a must for devices in 90nm or less technology nodes.

The question about working at various temperatures is certainly vital, not only due to the fact of varying ambient temperatures, but also because of internal device heating, which can be quite a trap for some unsuspecting designer trying to utilize the new part. A prime example of that problem is all of the very cheap controls using 4000 series CMOS and depending on the trigger threshold voltage being at some specific level for correct operation.

You Violet was pretty smart. In some phases of my engineering career, I really wanted to use HALT/HASS or some variation of it, but rarely got to do it. It was clear that adding thermal stress can reveal all kinds of weaknesses, in many kinds of products.

When it comes to this month’s ISSCC show, there are plenty of sections dedicated to solving issues with integrated analog technology. Here, our blogger looks at the sessions that offer new insight to difficult problems.

The International Solid State Circuits Conference kicks off February 9. While the mass media will focus on the biggest memory, faster processor, and smallest geometries reported, I prefer to look at the analog stuff.